The proposed research will explore the evolution of our human-like sense of fairness. To do so, we will examine the nature of inequity aversion in a closely-related primate species, the brown capuchin monkey (Cebus apella). The goal of our project will be to explore which of two proposed mechanisms govern capuchins' intuitions about inequity. Specifically, is inequity aversion governed by social cognitive processes related to reasoning about others' intentions or is instead to due more domain-general numerical cognitive skills? These goals will be achieved by studying trading behavior in capuchin monkeys, the standard primate model for inequity aversion due to their intelligence and prevalence of cooperation in their natural habitat. Monkeys will participate in two trading paradigms-inequity aversion and reference dependence. In inequity aversion studies we will measure the rate of trade observed when the experimenter rewards a second monkey with either the same reward (equity) or a better reward (inequity) than the subject received for the same trade. In reference dependence studies we will measure proportion of trading between two researchers-one who offers one reward (1 grape) and half the time adds to it (resulting in 2 grapes) and one who offers two rewards (2 grapes) and half the time subtracts from it (resulting in 1 grape). To explore social processing, we will manipulate the intentions of the researcher in these paradigms such that half the time the inequity or change in reward occurs intentionally and half the time accidentally. To manipulate numerical processing, we will vary the discriminability of the rewards, such that half the time the rewards will be easily discriminable and half the time they will not be. Our findings will provide insight into cognitive mechanisms necessary for fairness judgments and will therefore clarify the developmental trajectories that lead to an adult human sense of fairness. Our studies will thus also shed light on social cognition in clinical populations who may lack these reasoning capacities (i.e., autists who cannot reason about intentions, or patients with Down syndrome who have deficits in mathematical cognition).